Sarcopenia, the age-related skeletal
muscle decline, is associated with relevant clinical and socioeconomic
negative outcomes in older persons. The study of this phenomenon and
the development of preventive/therapeutic strategies represent public
health priorities. The present document reports the results of a recent
meeting of the International Working Group on Sarcopenia (a task force
consisting of geriatricians and scientists from academia and industry)
held on June 7–8, 2011 in Toulouse (France). The meeting was
specifically focused at gaining knowledge on the currently available
biomarkers (functional, biological, or imaging-related) that could be
utilized in clinical trials of sarcopenia and considered the most
reliable and promising to evaluate age-related modifications of
skeletal muscle. Specific recommendations about the assessment of aging
skeletal muscle in older people and the optimal methodological design
of studies on sarcopenia were also discussed and finalized. Although
the study of skeletal muscle decline is still in a very preliminary
phase, the potential great benefits derived from a better understanding
and treatment of this condition should encourage research on
sarcopenia. However, the reasonable uncertainties (derived from
exploring a novel field and the exponential acceleration of scientific
progress) require the adoption of a cautious and comprehensive approach
to the subject.

This article is also appearing in Journal of Frailty & Aging.

1
Introduction

One of the most recognized changes in
body composition with senescence is the loss of skeletal muscle mass.
This loss occurs even among physically active older persons, and it was
originally termed “sarcopenia” for the Greek words “flesh” and “loss” [1]. The age-related loss
in skeletal muscle mass is associated with substantial social and
economic costs and is characterized by impairments in strength,
limitations in function, and ultimately physical disability and
institutionalization [2–4]. In consideration of
the increased awareness of this syndrome and the continued rapid
development of therapeutic strategies to slow or reverse sarcopenia,
the International Working Group on Sarcopenia was convened to address
issues related to the successful conduct of clinical trials in this
area [5].
This task force, consisting of geriatricians and scientists from
academia and industry, met again in Toulouse, France in June of 2011 to
discuss the current state of the art in the development of biomarkers
to be utilized in clinical trials on sarcopenia. The purpose of this
meeting was to gain an understanding of the currently available
parameters that could be utilized in clinical trials of sarcopenia and
to discuss future research needs in this area. Specific topics that
were addressed include: review of current consensus definitions of
sarcopenia, the importance of muscle performance and quality,
biomarkers in other clinical states and chronic diseases, potential
biomarkers for sarcopenia, applications in clinical trials, and
recommendations for future studies.

2
Definition of sarcopenia

Since the advent of the term
“sarcopenia” in 1989, there has been a dramatic increase in
publications in this area and clinical interest in this condition [6]. Originally described
as the age-related decrease in skeletal muscle mass [7], until very recently,
there has been a lack of consensus on the operational definition of
sarcopenia without clinically appropriate correlates for this syndrome.
In the past 2 years, a number of academic societies have put forward
operational definitions of sarcopenia [8–11]. Although each
consensus definition has some distinct features, there is general
agreement among these groups on the definition of sarcopenia. A summary
of consensus sarcopenia definitions is presented in Table 1.
The characteristics of sarcopenia highlighted in these reports include
an objective measure of muscle or fat free mass using dual energy X-ray
absorptiometry (DXA) or computed tomography (CT), a reliable measure of
muscle strength, and/or an objective test of physical functioning.
Although the sequence of events and specific recommendations somewhat
differ, the general approaches proposed require that patients be
identified with measured deficits in physical function for which
sarcopenia may be the cause and subsequently quantification of muscle
strength and mass to definitively confirm the diagnosis.

3
Definition of a biomarker

A biomarker is defined as “a
characteristic that is objectively measured and evaluated as an
indicator of normal biological processes, pathogenic processes, or
pharmacologic responses to a therapeutic intervention” [12]. Hence, biomarkers
support the diagnosis, facilitate the tracking of changes over time,
and help clinical and therapeutic decision-making processes. Taking
this definition into account, the functional, biological, or
imaging-related parameters considered in the present document will be
hereby generally referred to with the term “biomarker.”

There are currently numerous parameters
that are potentially able to track the age-related skeletal muscle
decline. Depending on the parameter chosen to define sarcopenia,
different information might be obtained. Such variability depends on
the specific characteristics of each parameter and the mechanisms
measured by the parameter. The intrinsic (e.g., accuracy, specificity,
sensitivity) and extrinsic (e.g., cost, availability, time to be
performed) properties of each biomarker will largely drive its use in
research trials, making it more suitable for screening, baseline
evaluation, and/or definition of outcomes (Table 2).

- not recommended for this use; +
may be of use, but severely limited; ++ suitable for this use; +++
recommended for this use

aThe
importance of all of these biomarkers in the evaluation of sarcopenia
will largely depend on the study hypotheses, the specific aims, and/or
the target population

The use of biomarkers in a given study
must be “fit for purpose.” Thus, several different biomarkers may be
required to support different aspects of the development of a
therapeutic intervention. For example, biomarkers for detection and
diagnosis may not be the same as those that ideally track disease
progression. Likewise, for new therapeutic agents, a single assay may
not suffice as a biomarker reflecting both target engagement and the
pharmacodynamic effects of a drug.

4
Muscle quantity versus muscle quality

Although muscle mass can objectively
define the presence of sarcopenia, several components of skeletal
muscle function are not adequately captured by simply measuring mass or
cross-sectional area. It is now clear that there is a certain degree of
divergence between changes in muscle mass and alterations in muscle
performance. The well-described decline in skeletal muscle mass in
older adults is a critical determinant of age-related weakness, which
is defined as a reduction in maximal voluntary joint torque or power.
Yet, it is now clear that the relationship between force production
capability and muscle size in older adults is less robust than it is in
young people [13].
Indeed, longitudinal studies have demonstrated that the age-related
decline in muscle strength far exceeds the observed changes in muscle
mass or size, particularly in weight-stable individuals [14, 15]. Furthermore,
longitudinal studies indicate that maintenance or even gain of muscle
mass may not prevent weakness in older adults [15, 16]. In addition, a
number of age-related changes in force production capability are not
readily explained by a reduction in muscle mass, including decreased
specific force (force per cross sectional area) [17, 18] and slower rate of
isometric force production (expressed relative to peak torque or to
body weight) [19,
20].
Furthermore, voluntary weight loss leads to reductions in muscle
mass/size with no declines in muscle strength [21]. It is also
noteworthy that pharmacologic interventions that increase muscle
mass/size do not necessarily improve voluntary strength. Similarly,
physical activity interventions that increase muscle strength do not
necessarily augment muscle size [22,
23].
Noticeably, gains in muscle strength secondary to increased physical
activity generally precede measurable changes in skeletal muscle
mass/size.

The progressive muscle atrophy with
aging is associated with a loss of overall muscle force and changes in
force and power generation of the remaining muscle fibers [24]. However, several
additional physiological mechanisms that accompany the phenomenon of
sarcopenia may directly influence muscle function and force production
with advancing age. Recent evidence has shown that adipose tissue
accumulation around and between muscle fibers concomitant with
reductions in muscle cross-sectional area occurs with aging, and that
this skeletal muscle attenuation is inversely associated with muscle
performance [18,
25].
Age-related changes in the nervous system may also play a substantial
role in the decline in muscle power generation [26]. These include loss
of motor neurons and concomitant remodeling of motor units through
collateral reinnervation [27],
impairment of neuromuscular activation observed as decreased maximal
motor unit firing rates [28–30], and uncoordinated
patterns of intermuscular neural activation [31]. Finally, changes in
individual muscle fiber composition and intrinsic contractile
properties may influence the decline in muscle force among older
adults. For instance, cross-sectional observations suggest that
reductions in muscle torque may be related to changes in fiber
composition and, in particular, to the preferential atrophy of type II
(fast-twitch) fibers with aging [32].
Specific changes in the intrinsic ability of aged muscle to generate
force have also been observed [33].
Decreases in specific force (force normalized per cross sectional area)
and unloaded shortening velocity in type I and IIA fibers have been
reported in older males compared with young controls [32, 34]. Conversely, recent
longitudinal data have demonstrated that, despite reductions in whole
muscle cross-sectional area, single muscle fiber contractile function
is preserved with advancing age as existing fibers may compensate and
partially correct these deficits, therefore maintaining optimal
force-generating capacity [14].

Although precise and valid measures of
muscle mass are important components of sarcopenia assessment, these
gross measures of muscle size do not adequately account for the dynamic
components (force, power, activation) of muscle function that are
responsible for performing activities of daily living. Future trials on
sarcopenia adopting clinically meaningful endpoints should evaluate
these key biomarkers of muscle function through the use of
state-of-the-art methodologies.

5
Quantitative assessment of sarcopenia

The bidimensional definition of
sarcopenia simultaneously includes a functional parameter (i.e., muscle
performance) and a quantitative index (i.e., muscle mass). Therefore,
techniques aimed at capturing the objective amount of skeletal muscle
mass are required. Multiple methodologies are currently available to
accomplish this task [35].

DXA is the most commonly used imaging
technique for several reasons: first of all, because it is commonly
available in clinical and research settings, being relatively
inexpensive, sufficiently precise, and well-accepted by older persons.
Second, the initial operative definition of sarcopenia proposed by
Baumgartner and colleagues [3]
was based on appendicular lean mass measured by DXA. Later on, DXA was
used to provide alternative definitions of sarcopenia based on the
fat-adjusted residual method [36].
Nevertheless, it cannot be ignored that the first operative definition
is dated more than 10 years, and during this time, several steps
forward have been made in refining imaging techniques as well as
understanding the sarcopenia phenomenon.

The identification of the “gold
standard” for the quantitative evaluation of muscle mass in clinical
trials (which is currently lacking) should be based on criteria of
accuracy (i.e., the degree of conformity of a measure to a standard or
a true value), precision (i.e., the degree of refinement with which an
operation is performed or a measurement stated), reproducibility (i.e.,
the quality of being reproducible under the same operating conditions
over a period of time or by different operators), sensitivity to change
(i.e., the degree of being modified by interventions), and
accessibility (i.e., its usual availability in research and clinical
centers).

DXA currently represents the more
accessible technique for body composition assessment. It may accurately
provide estimates of lean, fat, and bone tissues in the entire body or
in specific regions. Moreover, it is inexpensive and quick to be
performed. The radiation exposure associated with DXA is low and highly
acceptable (about 1 mrem, a quantity similar to that of a 3-day
background). The main limitations of this imaging approach reside in
some analytical differences across manufacturers and models, and the
risk of biased results due to the low differentiation between water and
bone-free lean tissue.

CT accurately measures a direct physical
property of the muscle (e.g., cross-sectional area and volume). It also
allows the evaluation of muscle density (a parameter related to
intramyocellular lipid deposits) as well as subcutaneous and
intramuscular adipose tissue deposition. The radiation exposure
associated with this technique is higher (i.e., about 15 mrem) than
with DXA.

Magnetic resonance imaging (MRI)
presents a high agreement with CT and provides similar measures. It
does not involve radiation exposure and also has the additional
capacity of multiple slice acquisition, thus rendering 3D volumetric
estimates. The lack of radiation exposure makes MRI the method of
choice for many studies where ethics committee or national authority
approval is more difficult to obtain for CT. The major limitations of
this methodology reside in the higher technical complexity and costs,
and in the inapplicability to subjects with older models of implanted
metal devices (e.g., joint prostheses, pace-makers, etc.). Both CT and
MRI may be limited in the ability to accommodate very obese individuals.

Finally, it needs to be emphasized that
imaging provides information only about one of the two sarcopenia
dimensions. As discussed earlier, changes in muscle function and
quantity do not necessarily follow similar trajectories with aging [37]. Therefore,
interventions able to increase lean mass may not necessarily produce
parallel gains in strength and vice versa [38]. To overcome this
issue and include the two components of sarcopenia in the same
variable, it has been proposed to compute an index of skeletal muscle
quality derived from the ratio between strength and mass [15, 39, 40].

One of the most recently developed
techniques which might find larger application in the near future for
the evaluation of sarcopenia is the electrical impedance myography
(EIM) [41].
This is a noninvasive, painless approach based on the surface
application and measurement of a high-frequency, low-intensity
electrical current applied to specific muscles. EIM detects changes in
the conductivity and permittivity of skeletal muscle caused by
alterations in muscle composition and structure. EIM is repeatable and
sensitive to skeletal muscle changes in patients with amyotrophic
lateral sclerosis [42].
Moreover, its changes over time may also have clinical relevance as
they are predictive of survival in animal models of amyotrophic lateral
sclerosis [43].
Finally, it is also noteworthy that the EIM phase shows a consistent
inverse relationship with age [44].

An alternative method to measure
skeletal muscle size is by ultrasonography. This technique has shown to
be a valid (versus MRI-based measurements) and highly reliable way for
assessing cross-sectional areas of large individual human muscles [45]. It is particularly
useful in mobility-impaired subjects who cannot easily be transported
to scanners such as CT or MRI machines.

Also remarkable is the development of
mass isotopomer distribution analysis based on the evaluation of
protein and proteome synthesis rate obtained by heavy water labeling [46, 47]. Although this
technique can still be considered suitable mainly for research
settings, its flexibility and the large amount of information it
provides about a wide spectrum of proteins make it extremely promising.

Other techniques are also available to
detect sarcopenia, but their limited validation, low accuracy, and
difficult large-scale implementation discourage their use. For example,
bioelectrical impedance analysis (BIA) is a popular, very simple, and
low-cost technique, but its results are far from being accurate. The
BIA technique is based on the notion that tissues rich in water and
electrolytes are less resistant to the electrical passage than adipose
tissue. The BIA is therefore based on a single body resistance
parameter (not a direct measure of skeletal muscle), and its results
can be easily altered by fluid retention and health status in general.
For these reasons, a recent consensus paper by the Society of
Sarcopenia, Cachexia and Wasting Disorders has discouraged the use of
BIA for the assessment of sarcopenia [9].

6
Definition of critical thresholds

There is still resistance to accept
sarcopenia as a clinical condition despite its well-established
relationship with major health-related negative events (in particular,
mobility and physical disability) [8].
This issue might (at least partly) be explained by the current lack of
clinically relevant thresholds that distinguish normal from abnormal
values of skeletal muscle mass.

Several approaches can be adopted to
identify critical cut-points. A paradigmatic example potentially
lending support to the operative definition of sarcopenia might be
provided by the approach previously adopted to identify osteoporosis on
the basis of bone mineral density. In fact, approaches that have been
developed for bone and osteoporosis may serve well for skeletal muscle
and sarcopenia. The clinical definition of a specific condition (which
will consequently lead to the indication for treatment) might be based
on:

1.

A parallel clinical diagnosis. For
osteoporosis, diagnosis can be obtained by evaluating the presence of
vertebral fractures or deformities at the X-ray examination. Vertebral
fractures indicate decreased bone strength, regardless of bone mineral
density. It is well established that patients with vertebral fractures
present an increased risk of new events and therefore require
treatment. This approach is legitimate and may well work, but may find
some limitations when applied in primary prevention.

2.

A biological assessment. Given its
well-established association with fracture risk, bone mineral density
may represent the key parameter on which to rely to determine the
presence or absence of osteoporosis. However, bone mineral density
(like any other biological marker) exists as a continuous variable,
does not present a clear threshold, and is parallel to gradients of
risk. Although necessary to provide clinical relevance to biological
markers, any categorization will lead to a loss of information and will
inevitably introduce an “arbitrary” decision. For the definition of
osteoporosis, the cut-off defining the disease was arbitrarily set by a
committee which judged the -2.5 standard deviations at the T-score as
an adequate match between risk and prevalence. One major problem with
the bone definition that should not be repeated for sarcopenia is the
inclusion of osteopenia. Osteopenia (defined by a bone mineral density
T-score ranging between -1 and 2.5) encompasses about 50 % of the
female healthy population and has led to confusion and concerns among
policy-makers regarding the validity of a construct that cannot really
be considered abnormal. An approach consistent with this model has also
been adopted in the definition of other clinical conditions such as
anemia [48].

3.

The risk of adverse clinical outcomes.
The indication to treatment of a specific condition (e.g.,
osteoporosis) might be based on the evaluation of risk of events (i.e.,
fractures) resulting from the assessment of multiple factors (which may
even not include bone mineral density) [49].
This approach will not be exclusively based on the single evaluation of
a (potentially inaccurate and/or arguable) biomarker, but on a more
comprehensive screening and on cost-effectiveness analyses (e.g., treat
if the 10-year risk is exceeding a critical threshold). With this
rationale, the FRAX [50]
and QFractureScores [51]
algorithms were recently developed to guide osteoporosis treatment.

7
Biological markers of sarcopenia

Given the syndromic nature of
sarcopenia, intervention strategies aimed at preventing/treating its
process might need to target multiple risk factors. In this context,
several biological markers have been shown to be associated with
skeletal muscle mass, strength, and function, thus representing
potential markers for the effect of the studied interventions. Such a
list is quite long, and each biomarker identifies a specific mechanism
contributing the age-related skeletal muscle decline, although they are
not specific to muscle and many are likely to turn out to be only
weakly associated with clinically relevant outcomes. The most common
markers are inflammatory biomarkers [e.g., C-reactive protein [52, 53], interleukin-6 [52–54], and tumor necrosis
factor-α [52,
54]],
clinical parameters [e.g., hemoglobin [55,
56],
serum albumin [57,
58],
and urinary creatinine [59]],
hormones [e.g., dehydroepiandrosterone sulfate [60], testosterone [61], insulin-like growth
factor-1 [62],
and vitamin D [63–65]], products of
oxidative damage [e.g., advanced glycation end-products [66], protein carbonyls [67, 68], and oxidized
low-density lipoproteins [69]],
or antioxidants [e.g., carotenoids [70,
71] and
α-tocopherol [70]].

Other promising biomarkers have been
identified in the last years and may represent useful parameters to
more directly explore sarcopenia because they are closely related to
skeletal muscle changes. For example, plasma concentrations of
procollagen type III N-terminal peptide (P3NP) represent an interesting
marker of skeletal muscle remodeling [72,
73].
P3NP is a fragment released by the cleavage of procollagen type III to
generate collagen III (a protein produced in soft connective tissues,
skin, and muscle). Preliminary studies have also suggested an
interesting role played by biomarkers specifically linked to the
neuromuscular junction in evaluating skeletal muscle modifications [74, 75].

8
Clinical outcome measures of sarcopenia

Ultimately, the goal of clinical trials
for sarcopenia treatments will require the evaluation of clinical
benefit. In fact, clinical measures can also be considered as
biomarkers as they reflect the impact of the pathological process of
sarcopenia on the patient's health. The assessment of measures of
muscle strength (e.g., hand grip), muscle power (e.g., leg extension
power), and physical performance [e.g., Short Physical Performance
Battery [4]
and gait speed tests] comprise important indices of the individual's
physical function. In addition, functional outcome measures will need
to be developed in order to help understand the impact of any
treatment-related quantitative gains in performance on the person's
daily life.

9
Recommendations

9.1
Adoption of comprehensive operative definitions

The lack of a unique operative
definition of sarcopenia and the numerous methodological issues could
potentially hinder efforts to study sarcopenia and to develop effective
treatments. Such difficulties should not hamper the process of
exploring this syndrome which severely affects the health status of
millions of older persons. The current ambiguities can be easily
overcome by adopting flexible and comprehensive approaches in the
design of studies, for example, by avoiding reliance on a single
parameter or technique to evaluate age-related skeletal muscle decline.
The adoption of a variety of assessment approaches in combination is
agreeable. Although this might lead to the risk of conflicting results
(and increase the need of resources), it will serve to (1) capture
different domains of the sarcopenia syndrome, (2) provide useful
insights about the pathophysiological process underlying this
phenomenon, and (3) facilitate the development and use of the findings
in future and more definitive studies. In this context, it is
noteworthy the lack of studies simultaneously testing different
techniques measuring skeletal muscle (e.g., MRI, CT, DXA, etc.) in
relationship with clinically meaningful outcomes. Such studies might
greatly help in the standardization of instruments and in the adoption
of an univocal direction in the study of sarcopenia.

9.2
MRI and CT scan to be equally considered as “gold
standard” imaging techniques

It is now clear that, to be adequately
assessed, the sarcopenia phenomenon cannot merely rely on the
evaluation of the contractile part of skeletal muscle. The close
relationship between lean mass and adipose tissue in determining
age-related decline of skeletal muscle is evident [38, 76, 77]. Therefore,
techniques allowing the simultaneous evaluation of fat and muscle
should be preferred. DXA, CT, and MRI are the most important assessment
instruments. CT and MRI should be considered the “gold standard”
techniques. The balance of pros and cons for both CT and MRI does not
allow a clear indication on which of the two should be preferred.
Resources, instrument availability, and need of details will represent
the factors guiding the investigator's preference for one over the
other. On the other hand, DXA should not be discarded and still
represents the instrument more likely to promote the “clinical
relevance” of sarcopenia. For its characteristics, DXA may be an
extremely interesting methodology to be used for preliminary screening.
Moreover, its use in combination with either CT or MRI will help drive
the research in the field towards more clinical aspects. While imaging
and other biomarkers will be valuable tools for initial proof of
concept studies, assessment tools for evaluating the effect of
treatments on outcomes reflecting clinical benefit will be required to
support eventual pivotal studies.

9.3
Adequate length of study

To evaluate the efficacy of a specific
intervention on sarcopenia, it is necessary that the follow-up will be
sufficiently long to allow the hypothesized modifications of
biomarkers. Surely, not all biomarkers will be similarly influenced by
the intervention. Such variations will depend on multiple factors,
including the population characteristics, the type and strength of the
tested intervention, and the sensibility of the biomarker to changes.
However, 6 months have been generally indicated as the minimum
timeframe to expect changes in imaging parameters.

9.4
Sarcopenia is a “work in progress”

The study of sarcopenia is still in its
infancy, but we have clearly acknowledged the great potential benefits
arising from the understanding and treatment of this condition at both
person and population levels. Taking together the uncertainties of
exploring a novel field with the exponential acceleration of scientific
progress, it is currently difficult to provide long-lasting statements,
recommendations, and guidelines. It is likely that what seems
reasonable today will be confounded by several studies in the near
future. For this reason, extreme caution is needed to avoid
jeopardizing the future development of research in the field. It is
important to consider the study of sarcopenia as a “work in progress,”
always amenable to changes and redirections. After all, the first phase
II trials in this syndrome are just starting, and this is the
appropriate time to raise doubts and pose questions. With time, a
stronger foundation for sarcopenia research will be developed, which
will ultimately lead to larger scale and more definitive studies. In
this context, it is critical that an ongoing dialogue be initiated and
sustained among investigators with an interest in age-dependent decline
of muscle.

Acknowledgements

Dr. Fielding's contribution is
based upon the work supported by the US Department of Agriculture,
under agreement No. 58-1950-7-707. The authors of this manuscript
certify that they comply with the ethical guidelines for authorship and
publishing in the Journal
of Cachexia, Sarcopenia and Muscle [78].

MC has received consultancy fees from
Sanofi-Aventis and Pfizer; RAF is a consultant with Merck, Eli Lilly,
Cytokinetics, DMI, Kraft Foods, and Unilever; MH is a stockholder,
chairmen of scientific advisory board, and consultant for KineMed,
Inc.; SA is a consultant with Brahms, Vifor, Professional Dietetics,
PsiOxus, and Takeda; receives research support from Vifor and BG
Medicine; and has received fees for speaking at meetings from Brahms
and Vifor; SR has equity in and receives consulting income from
Convergence Medical Devices, Inc; WV is an employee and a shareholder
of Neurotune AG; YR receives support from Lactalis, Lundbeck, Lilly,
Nutricia, Servier, Cheisi, Ipsen, and Novartis; JMC is an employee and
a shareholder of Cytokinetics, Inc; MZ has received a fee from Abbot
for a conference; DL and RR are employed by Novartis; WJE is employed
by GlaxoSmithKline; JEM is a consultant and a stokeholder of Mattern
Pharmaceuticals and a consultant for Sanofi-Aventis; BV is a consultant
and a member of the Advisory Board with Novartis, Servier, and Nestlè.
MP, BG, GAVK, MI, CMR, MAL, and CCS have no conflict of interest to
declare.

Disclaimer

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and do not necessarily reflect the position of the supporting
organizations or agencies.

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